From a view point of energy saving, a power supply unit of an isolation type DC-DC converter is required to have a high efficiency over a broad load range. Because of this, while using a power supply unit including a full bridge circuit at the primary side and a current doubler type rectifier and smoothing circuit at the secondary side, and applying thereto a soft switching technology that realizes a ZVS (Zero Volt Switching) by making use of a resonant inductor Lr and a resonant capacitor Cr in the full bridge circuit, a high efficiency thereof has been achieved. However, the power supply units of this type had a shortcoming in decreasing efficiency during a light load because the ZVS cannot be realized during such light load. The following examples are exemplified as conventionally known methods for solving this problem. Both intend to realize the ZVS with a method of increasing the value of the resonant inductor during a light load.